Self-interference suppression control for a relay node

ABSTRACT

A technique for controlling suppression of self-interference in a relay node configured to transmit and receive simultaneously using the same frequency channel or using proximate frequency channels is provided. A method implementation of this technique comprises the steps of actively cancelling a signal transmitted from the relay node that leaks back into a receiver of the relay node to suppress self-interference, determining whether an increase of an amount of self-interference suppression is needed or whether self-interference suppression can be decreased, and increasing or decreasing, depending on the result of the determination, at least one of the transmit power of the signal transmitted from the relay node and the receive power of the relay node.

TECHNICAL FIELD

The present disclosure generally relates to a technique for controllingsuppression of self-interference in a relay node. In particular,self-interference suppression control for a relay node configured totransmit and receive simultaneously using the same frequency channel orproximate frequency channels is described.

BACKGROUND

A radio access connection in a cellular communication system typicallystretches between a base station and a user terminal served by the basestation. A relay node is intended to give increased coverage (i.e., toextend the radio access connection) without the need to install afurther base station.

When a relay node is included in a radio access connection between abase station and a user terminal, the base station is referred to as adonor base station. The communication link stretching between the donorbase station and the relay node is referred to as backhaul link (alsodenoted Un), whereas the communication link stretching between the relaynode and the user terminal is referred to as the access link (alsodenoted Uu).

A relay node has two transmitters and two receivers, one each for thebackhaul link and the access link. In general, care has to be taken whenoperating the two links in order to ensure that they do not interferewith one another. In particular, it has to be ensured that the receiverfor one link does not experience strong interference from thetransmitter for the other link (“self-interference”).

FIG. 1 illustrates an exemplary communication scenario involving a basestation 10, a user terminal 20 and a relay node 30 extending a radioaccess connection 40 between the base station 10 and the user terminal20. The relay node 30 defines a relay cell 50 in which the user terminal20 is served. In the downlink (DL) from the base station 10 to the userterminal 20, the relay node 30 receives data from the base station 10,decodes and re-encodes it, and then transmits the re-encoded data to theuser terminal 20. In the uplink (UL) the same steps are performed by therelay node 30, but in the other direction from the user terminal 20 tothe base station 10. The relay node 30 can generally have a similaroutput power as the base station 10. Still, there are many deploymentscenarios in which a significantly lower output power of the relay node30 will suffice.

One alternative to a relay node is a repeater. In a repeater, the datais not decoded and re-encoded, but rather the signal is just amplifiedand re-transmitted. For this reason repeater operation is also referredto as Amplify and Forward (AF), whereas relay operation is commonlyreferred to as Decode and Forward (DF).

A repeater will be faced with the problem that it needs to receive arather weak signal at the same time as it is transmitting a signal thatis considerably stronger. While the power of the received signal mightbe in the order of −80 dBm, the power of the transmitted signal might bein the order of 0 dBm. To avoid self-interference and a resultingself-oscillation, this puts rather hard requirements on the amount ofisolation required. As a rule-of-thumb, the isolation should be about 10dB higher than the amplification of the signal. For instance, if theamplification is 80 dB as in the example above, then the isolationshould be about 90 dB.

In Eun-Ji Yoo et al., “Self-Interference Cancellation Method based onV-BLAST in MIMO Systems”, ICACT 2009, p. 800-803, Feb. 15-18, 2009 (ISBN978-89-5519-139-4) a technique for cancelling self-interference in arepeater is described. The technique comprises the establishment of afeedback channel between an input and an output of an amplifier of therepeater. In addition, an adaptive filter is provided that is based on aLeast-Mean-Square (LMS) algorithm for cancelling self-interference.

It has been found that there is a crucial difference between a repeaterand a relay node when it comes to the fundamental mechanisms underlyingself-interference. Since a repeater does not decode the information, therequirements for self-interference suppression will be set by thequality of the transmitted (amplified) signal. For a relay node, on theother hand, the received signal needs to be decoded, which means thatthe requirements will be determined by the requirements of the receiverin the relay node.

There are two different approaches for a relay node to avoid or at leastsignificantly reduce interference, and in particular self-interference.According to a first approach, the two links use different frequencybands (out-of-band relaying), in which case coexistence of the two linksis ensured by means of filtering. According to a second approach, thetwo links use different time slots in a frame structure so thatcoexistence is ensured by means of scheduling (in-band relaying).

The second approach, which is based on the use of different time slots,implies restrictions on the maximum data rate as well as on thescheduling of the individual transmissions. One drawback of the firstapproach is the fact that different frequency bands are needed for thetwo links. This need implies that twice the amount of spectrum isrequired for operating both links (compared to a communication scenariowithout a relay node). Also, the frequency separation between the twolinks must be rather large to make filtering feasible. If the separationis not large enough, so that no filtering can be applied in thereceivers of the relay node, the relay node will experience strongself-interference.

SUMMARY

There is a need for a relay node that is less susceptible toself-interference.

According to one aspect, a relay node configured to control suppressionof self-interference is provided, wherein the relay node comprises atransmitter and a receiver and wherein the relay node is adapted totransmit and receive simultaneously using the same frequency channel orusing proximate frequency channels. The relay node also comprises aninterference signal estimator having a first input adapted to receive atransmitter signal from the transmitter, a second input adapted toreceive adaptation metric, and an output adapted to output an estimatedinterference signal generated by the interference signal estimator basedon the transmitter signal and the adaptation metric, and a subtractorcoupled to the output of the interference signal estimator andconfigured to subtract the estimated interference signal from a receivedsignal in the receiver so as to actively cancel a signal transmittedfrom the relay node that leaks back into the receiver of the relay nodeto suppress self-interference.

The first input of the interference signal estimator may be coupled to adigital baseband stage of the transmitter. The first input may, inaddition or as an alternative, also be coupled to other stages of thetransmitter.

In case the first input of the interference signal estimator is coupledto the digital baseband stage of the transmitter, one or morereplication blocks coupled between the first input of the interferencesignal estimator and the digital baseband processing stage, or includedin the digital baseband processing stage, may be provided. The one ormore replication blocks may be configured to replicate (or model) errorsintroduced by a transmitter chain in the transmitter signal supplied tothe interference signal estimator. Specifically, the one or morereplication blocks may comprise at least one of a filter replicationblock adapted to replicate (or model) errors introduced by analoguefilters, an In-phase/Quadrature-phase (IQ) modulator replication blockadapted to replicate (or model) errors introduced by at least one of IQimbalance and Direct Current (DC) offset, and a power amplifierreplication block adapted to replicate (or model) errors introduced by apower amplifier.

In one implementation, the relay node is specifically adapted totransmit and receive simultaneously using proximate frequency channels(e.g., using frequency channels located within a single frequency band).In such a realization, the relay node may further comprise a frequencytranslator coupled between the first input of the interference signalestimator and the digital baseband processing stage. The frequencytranslator may be adapted to shift the transmitter baseband transmittersignal such that it reflects the difference in local oscillatorfrequency used for the receiver and the transmitter of the relay node.

The relay node may further comprise a measurement receiver coupled tothe first input of the interference signal estimator. The measurementreceiver may be adapted to generate equivalent digital basebandcomponents of the transmitted signal. Alternatively, or in addition, therelay node may comprise a vector gain processing block coupled betweenan output of a power amplifier of the transmitter and an input of aradio frontend of the receiver. The vector gain processing block may beadapted to provide an at least coarse cancellation of the transmittedsignal leaking back into the receiver. This may be viewed as if, e.g.,one tap of an adaptive filter is implemented in the analogue domain,whereas the remaining taps of the adaptive filter are implemented in thedigital domain.

The adaptation metric may generally be based on one or more items ofmetric information. The metric information may generally be indicativeof an interference level. In this regard, the adaptation metric may, forexample, be based on a Signal-to-Noise Ratio (SNR), a Bit Error Rate(BER), or a similar parameter.

The relay node may further comprise a component adapted to determinewhether an increase of an amount of self-interference suppression isneeded or whether self-interference suppression can be decreased, aswell as a component adapted to increase or decrease, depending on theresult of the determination, at least one of a transmit power of thesignal transmitted from the relay node and a receive power of the relaynode. Still further, a component adapted to request a decrease orincrease of a transmit power of a signal received by the relay node todecrease or increase the receive power may be provided.

The interference signal estimator may be configured in various ways. Inone implementation, the interference signal estimator is configured asan adaptive filter or comprises an adaptive filter. In such arealization, the relay node may further be configured to determine anamount of suppression needed and to adapt a complexity (e.g., a length)of the adaptive filter based on the amount of the suppression needed.One reason to adapt the complexity is because of power consumption. Incase it suffice using a shorter filter, this is preferable because lesspower is consumed compared to if a longer filter would be used. Anotherreason to adapt the complexity might be convergence speed of the filter.Typically, a shorter filter will converge faster than a longer one.Therefore, in case the channel for the interfering signal is changing sothat fast tracking of the adaptive filter is required, a less complexfilter might be needed.

The interference signal estimator may comprise a first estimation blockconfigured to estimate a blocking part and a second estimation blockconfigured to independently estimate a co-channel interference part ofthe interference signal. In such an implementation, the subtractor maybe configured to subtract both the estimated blocking part and theestimated co-channel interference part from the received signal.

Also provided is a method of controlling suppression ofself-interference in a relay node, wherein the relay node comprises atransmitter and a receiver and wherein the relay node is adapted totransmit and receive simultaneously using the same frequency channel orusing proximate frequency channels, wherein the method comprisesestimating an interference signal based on a transmitter signal from thetransmitter and adaptation metric, and subtracting the estimatedinterference signal from a received signal in the receiver so as toactively cancel a signal transmitted from the relay node that leaks backinto the receiver of the relay node to suppress self-interference.

The estimating step may be performed by adaptive filtering or any otherestimating approach. The complexity of the adaptive filtering (e.g., afilter length) may be adapted as needed.

The method may further comprise replicating (or modeling) errorsintroduced by a transmitter chain in the transmitted signal supplied toor for interference signal estimation. As an example, one or more of thefollowing errors may be replicated (or modeled): errors introduced byanalogue filters, errors introduced by at least one of IQ imbalance andDC offset, and errors introduced by a power amplifier.

In the case adaptive filtering is employed, the method may furthercomprise running the adaptive filtering for a predetermined amount oftime, or until a termination condition has been fulfilled. In a nextstep one or more processing blocks in a transmitter chain or one or moreblocks conveying the transmitter signal to interference signalestimation are adapted or calibrated. At this stage, the method returnsto the adaptive filtering for a predetermined amount of time, or untilthe termination condition has been fulfilled.

The relay node may be adapted to transmit and receive simultaneouslyusing proximate frequency channels. In such an implementation, thetransmitter signal for interference signal estimation may be atransmitter baseband signal, and a method may further comprise shiftingthe transmitter baseband signal before interference signal estimationsuch that it reflects the difference in local oscillator frequency usedfor the receiver and the transmitter of the relay node.

The method may comprise providing a measurement receiver and generatingequivalent digital baseband components of the transmitted signal by themeasurement receiver. The equivalent digital baseband components of thetransmitted signal may constitute the transmitter signal forinterference signal estimation.

Additionally, or as an alternative, the method may further compriseproviding a vector gain processing block coupled between an output of apower amplifier of the transmitter and an input of a radio frontend ofthe receiver. With such an implementation, the method may compriseoperating vector gain processing block to provide an at least coarsecancellation of the transmitted signal leaking back into the receiver.

The step of estimating the interference signal may compriseindependently estimating a blocking part and a co-channel interferencepart of the interference signal. Both the estimated blocking part andthe estimated co-channel interference part may be subtracted from thereceived signal.

According to another aspect, a computer program product comprisingprogram code portions is provided. The program code portions may beconfigured for performing the steps of any of the methods and methodaspects disclosed herein when the computer program product is executedby a computing device, such as a processor of a relay node. The computerprogram product may be stored on a computer-readable recording mediasuch as a CD-ROM, DVD-ROM, semi-conductor memory and so on. The computerprogram product may also be provided for download via a wired orwireless communication connection.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, aspects and advantages of the present disclosure willbecome apparent from the following description of exemplary embodimentsin conjunction with the drawings, wherein:

FIG. 1 schematically illustrates a portion of a cellular communicationsystem comprising a base station, a user terminal as well as a relaynode in which embodiments of the technique presented herein can beimplemented;

FIGS. 2A and 2B illustrates the transmitter and receiver chains of relaynodes in which embodiments of the technique presented herein can beimplemented;

FIG. 3 illustrates an embodiment of a relay node;

FIG. 4 is a flow diagram illustrating a method embodiment;

FIG. 5 illustrates a still further embodiment of a relay node;

FIG. 6 is a flow diagram illustrating another method embodiment;

FIGS. 7-10 are diagrams illustrating variation of the effective SNRdependent on filter adaptation according to embodiments of the presentdisclosure; and

FIGS. 11-18 illustrate further embodiments of relay nodes.

DETAILED DESCRIPTION

In the following description of exemplary embodiments, for purposes ofexplanation and not limitation, specific details are set forth, such asspecific relay node configurations and cellular network types, toprovide a thorough understanding of the present disclosure. It will beapparent to one skilled in the art that the technique presented hereinmay be practised in other embodiments that depart from these specificdetails. The skilled person will appreciate, for example, that while thefollowing embodiments will partially be described in terms as used inthe Long Term Evolution (LTE) project and in particular Release 10 ofthe 3^(rd) Generation Partnership Project (3GPP) standards, according towhich the physical layer processing is based on Orthogonal FrequencyDivision Multiplexing (OFDM), the base station is referred to as eNodeBand the user terminal is referred to as User Equipment (UE), thetechnique presented herein is not limited to being implemented accordingto any specific 3GPP or non-3GPP standard.

Moreover, those skilled in the art will further appreciate that themethods, steps and functions discussed herein may be implemented usingindividual hardware circuitry, using software functioning in conjunctionwith a programmed microprocessor or a general purpose computer, usingone or more Application Specific Integrated Circuits (ASICs), one ormore Digital Signal Processors (DSPs) and/or one or more FieldProgrammable Gate Arrays (FPGAs). It will also be appreciated that themethods, steps and functions disclosed herein may be embodied in aprocessor and a memory coupled to the processor, wherein the memorystores one or more programs that perform the steps discussed herein whenexecuted by the processor.

FIGS. 2A and 2B illustrates various components of a relay node 30 inwhich the technique presented herein may be implemented. As illustratedin FIGS. 2A and 2B, the relay node 30 comprises a first transceiver forthe Un link in FIG. 1 (Un transceiver 21) as well as a secondtransceiver for the Uu link (Uu transceiver 22). Each transceiver 21, 22comprises a receiver (RX) as well as a transmitter (TX). Additionally,for each of the Un transceiver 21 and the Uu transceiver 22, anassociated duplexer 23, 24 coupling the respective transceiver 21, 22 toan associated antenna 25, 26 is provided.

The relay node 30 presented herein is generally configured to transmitand receive simultaneously using the same frequency channel or usingproximate frequency channels. As understood in the context of thefollowing exemplary embodiments, proximate frequency channels aregenerally not any further apart than what can be accommodated in asingle frequency band. As understood herein, a frequency band refers toa RX and/or TX band (e.g., as specified by 3GPP). Generally, in aproximate frequency channel scenario, interference at a receiver cannotbe suppressed by a duplexer or RX filter as such components only removewhat is outside the respective band. In absolute numbers, the width ofthe frequency band, and thus the spacing of the proximate frequencychannels, may vary. As an example, a bandwidth of 70 MHz is allocatedfor the UL and DL, respectively, in band 7 (see, e.g., 3GPP TS 36.104).

As illustrated in FIG. 2A, for proximate frequency channel operationself-interference caused by leakage of the transmitted signal into thereceived signal can be divided into two contributions, or parts:blocking interference (“blocker”) and co-channel interference. The mainlobes of the transmitter signals act as blockers that at least consumehighly valuable dynamic range of the receivers or, if the blocking levelis high enough, even force the receivers into compression. Due todistortion and other spectral emission contributions, the transmittersignals will broaden into the channel of the received signal, whichleads to co-channel interference. Blocking interference as well asco-channel interference are schematically illustrated in the signalsreceived by the relay node 30 at the Un side as well as the Uu side inthe frequency diagrams of FIG. 2A.

FIG. 2B illustrates the self-interference situation in an operationalmode in which the relay node 30 is configured to transmit and receivesimultaneously using the same frequency channel. Compared to theoperational mode illustrated in FIG. 2A, the blocking interference willbecome co-channel interference when the same channel is used for bothlinks (e.g., when both links are based on Time Division Duplex, TDD).

The technique presented herein targets at suppressing at least one ofblocking interference and co-channel interference as illustrated inFIGS. 2A and 2B, and to that end the relay node 30 as illustrated inFIGS. 1, 2A and 2B is equipped with dedicated components for controllingself-interference suppression as illustrated in FIG. 3.

FIG. 3 shows an embodiment of a relay node 30 with the components forimplementing the technique presented herein. It should be noted that therelay node 30 may comprise further components, for example asillustrated in FIGS. 2A and 2B.

With reference to FIG. 3, the relay node 30 comprises a canceller 52, adetermination component 54, a power control component 56 as well as anoptional power change requestor 58. The canceller 52, which may beconfigured as an echo canceller, is adapted to actively cancel a signaltransmitted from the relay node 30 that leaks into a receiver (denotedby RX in FIGS. 2A and 2B) of the relay node 30 to suppressself-interference. The canceller 52 may be based on an adaptive filter.In such a case, the relay node 30 may be further configured to determinean amount of suppression needed and to adapt a complexity of theadaptive filter based on the amount of suppression needed.

The determination component 54 is adapted to determine whether anincrease of an amount of self-interference suppression is needed, orwhether self-interference suppression can be decreased. Thedetermination may be based on a quality of a signal received by therelay node 30 (e.g., in terms of a Signal-to-Noise Ratio, SNR, a BitError Rate, BER, or any other metric indicative of an interferencelevel). Depending on the result of the determination by thedetermination component 54, and optionally depending on the availablepower margins, the power control component 56 increases or decreases atleast one of a transmit power of the signal transmitted from the relaynode 30 and a receive power of the relay node 30 (e.g., for a signalreceived from the base station 10 or the user terminal 20 in FIG. 1).

The power change requestor 58 is adapted to request a decrease orincrease of a transmit power of a signal received by the relay node 30so as to decrease or increase the receive power of the received signal.The transmit power of the received signal will typically be adjusted byeither the base station 10 or the mobile terminal 20, so that the powerchange requestor 58 will communicate with either the base station 10 orthe user terminal 20 when a decrease or an increase of the receive powerat the relay node 30 is needed, desired or possible.

The operation of the relay node 30 according to an exemplary methodembodiment is illustrated in the flow diagram 400 of FIG. 4.

In a first step 402, a signal transmitted from the relay node 30 thatleaks back into a receiver of the relay node 30 is actively cancelled tosuppress self-interference. Step 402 may be performed by the canceller52 illustrated in FIG. 3.

In a next step 404 it is determined whether an increase of an amount ofself-interference suppression is needed or whether self-interferencesuppression can be decreased. Step 404 may be performed by the powercontrol component 56 illustrated in FIG. 3, optionally in cooperationwith the power change component 58.

In a further step 406, a transmit power of the signal transmitted fromthe relay node 30 and/or a received power of the relay node 30 isincreased or decreased depending on a result of the determination instep 404. Step 406 may be performed by the power control component 56illustrated in FIG. 3.

The power control aspects described with reference to FIGS. 3 and 4 arebased on the insight that there often exist power margins that can beexploited for controlling suppression of self-interference. In thefollowing, the magnitudes of such power margins and the resultingpossibilities to increase or decrease a transmit power of the signaltransmitted from the relay node 30 and/or a received power of the relaynode 30 will be described with reference to specific examples.

Referring to FIG. 1, it can readily be seen that a good connectionbetween the base station 10 (e.g., an eNodeB) and the user terminal 20(e.g., a UE) requires both that the Un link and the Uu link are good.These two concatenated links should allow for a better transmission thanwould be the case in the absence of the relay node 30.

Because the relay node 30 typically is stationary and favourably placed,it can use directional antennas. In particular for the backhaul linkbetween the relay node 30 and the base station 10, the antennas can bedirectional since also the base station 10 is fixed. Although it mightnot be feasible to make the antenna in the access link between the relaynode 30 and the user terminal 20 directional, it can readily be seenthat it should not be omni-directional. If the relay node 30 forinstance is placed indoors, the antenna might be placed in a corner atthe ceiling, so that effectively it will only radiate in a limiteddirection.

Considering the access link more in detail, it is readily realized thatthe transmission power (“TX power”) of the relay node 30 in the accesslink will largely depend on the location of the user terminal 20.Analogously, supposing the user terminal 20 transmits at maximum powerirrespectively of the quality of the access link, the received signalpower will also largely depend on the location of the user terminal 20.

Considering the backhaul link, and the fact that highly directionalantennas might be employed, it can be expected that this link might havesome margin compared to if a somewhat arbitrarily placed user terminal20 without directional antennas would need to be supported at the samedistance from the base station 10.

In the following, it is exemplarily supposed that

-   -   the maximum TX power from the base station 10 is 43 dBm    -   the maximum TX power from relay node 30 is 30 dBm        -   The power in a proximate channel used by receiver is assumed            to be −40 dBc (in case of proximate frequency channel            operation)    -   the maximum TX power from the user terminal 20 is 23 dBm    -   the Noise Figure (NF) is 7 dB in all involved receivers of the        relay node 30, so that the thermal noise floor is at −107        dBm/MHz    -   the maximum bandwidth to be used is 20 MHz

The DL will be considered first, i.e., the connection from the basestation 10 to the relay node 30 and from the relay node 30 to the userterminal 20. It is supposed that the reason for including the relay node30 is to extend the coverage, or to increase the supported data rate fora user terminal 20 at the cell edge. This could for instance mean thatthe relay node 30 is located in a position where the received signallevel is, say, 5 dB below what is required to achieve the desired linkquality. A reasonable number for the required received signal powercould be −100 dBm/MHz. This corresponds to a Carrier to Noise powerratio (C/N) of 7 dB and implies that the path loss experienced for theuser terminal 20 is 43 dBm−(−100 dBm)+5 dB=148 dB.

Assuming that the relay node 30 can be placed in a favourable position,and also invoking directional antennas for the backhaul link, it issupposed that the effective path loss on the Un link is 100 dB, whereasthe effective path loss on the access link is 80 dB. With effective pathloss it is meant that the gain obtained from directional antennas isincluded so that in case of omni-directional antennas, the path losswould be increased.

Clearly, in case the maximum TX power is used by both the base station10 and the relay node 30, the received power will be 43 dBm−100 dB=−57dBm at the relay node 30 and 30 dBm−80 dB=−50 dBm at the user terminal20, respectively. With the assumption that −80 dBm/MHz was needed toreach an acceptable level of performance, this means that even with a 20MHz bandwidth (13 dB), there is a power margin of −50 dBm−13 dB−(−80dBm)=17 dB to the sensitivity level at the user terminal 20 and a powermargin of −57 dBm−13 dB−(−80 dBm)=10 dB to the sensitivity level at therelay node 30. With the assumption that −100 dBm/MHz was needed to reachan acceptable level of performance, this means that even with a 20 MHzbandwidth (13 dB), there is a margin of −50 dBm−13 dB−(−100 dBm)=37 dBto the sensitivity level at the user terminal 20 and a margin of −57dBm−13 dB−(−100 dBm)=30 dB to the sensitivity level at the relay node30.

In one embodiment, the margins for the two links are exploited in orderto simplify using proximate frequency channels or using the samefrequency channel for the links by the relay node 30. Specifically,since the issue with using proximate frequency channels or the samefrequency channel on the links is that the signal transmitted by therelay node 30 leaks into the receivers (denoted by RX in FIGS. 2A and2B) of the relay node 30, the signal transmitted by the relay node 30 isreduced as much as the link margin on the Uu link allows, e.g., 10 dB or30 dB in the examples above, whereas the transmitted power by the basestation 10 is not reduced (e.g., is maintained) in order to betterhandle the interference situation at the relay node 30.

Considering the interference situation for the DL at the relay node 30,a reasonable attenuation from the TX antenna 26 to the RX antenna 25(see FIGS. 2A and 2B) at the relay node 30 is 40 dB to 60 dB. Here wemake the conservative assumption of 40 dB. This means that theinterference power reaching the RX antenna 25 at the relay node 30 is(30 dBm−17 dB)−40 dB=−27 dBm (or −47 dBm in the other example givenabove), implying a C/B (Carrier to Blocker power ratio) of −30 dB (−10dB) and C/I (Carrier to Interference power ratio) of −30 dB+40 dB=10 dB(−10 dB) at the RX antenna 25. The 40 dB in the C/I computation refersto the 40 dBc out-off-band skirts of the TX signals in the proximatefrequency case (as it is distinguished here between blockinginterference and co-channel interference).

In case the link margin would not have been exploited, either byallowing the base station 10 to reduce its TX power or by not reducingthe TX power of the relay node 30, the C/I and C/B at the RX antenna 25of the relay node 30 could have been tens of dB worse, i.e., C/I=10dB−17 dB=−7 dB (or, in the other example, if a worsening of 30 dB isassumed, C/I=−40 dB). While C/I=10 dB is sufficient for correctlydecoding the information for some signal formats, a C/I of −7 dB orlower (e.g., −40 dB) is more challenging. Moreover, in certainsituations a C/I of −10 dB is not sufficient for correctly decoding theinformation, so that additional measures (such as active interferencecancellation) may be employed as will be discussed below.

Next, the UL is considered, i.e., the connection from the user terminal20 to the relay node 30 and from the relay node 30 to the base station10. Again, it is supposed that the reason for including the relay node30 is to extend the coverage, or increase the supported data rate for auser terminal 20 at the cell edge. Assuming the same path-loss for theUL as for the DL in the example above, the power margins obtained byinvoking a relay node 30 can once again be calculated.

In case the maximum TX power is used by both the user terminal 20 andthe relay node 30, the received power will be 30 dBm−100 dB=−70 dBm atthe base station 10 and 23 dBm−80 dB=−57 dBm at the relay node 30,respectively. With the assumption that −100 dBm/MHz was needed, thismeans that even with a 20 MHz bandwidth, there is a power margin of −70dBm−13 dB−(−100 dBm)=17 dB to the sensitivity level at the base station10 and a power margin of −57 dBm−13 dB−(−100 dBm)=30 dB at the relaynode 30. According to one embodiment, these power margins are exploitedsuch that the relay node 30 experiences as favourable conditions aspossible when it comes to self-interference. For the UL, this impliesthat the TX power for the backhaul link is reduced by 17 dB, whereas theTX power for the access link (the TX power by the user terminal 20) isnot reduced (e.g., kept at its maximum value).

Considering the interference situation for the UL at the relay node 30,again assuming a 40 dB attenuation between the two antennas 25, 26 ofthe relay node 30, it follows that the interference power reaching theRX antenna 26 will be (30 dBm−17 dB)−40 dB=−27 dBm, implying a C/B of−30 dB and C/I of 10 dB (or of −30 dB in the other example) at the RXantenna 26. If the transmitter output power would not have been reduced,or if the TX power of the user terminal 20 would have been reduced, theC/I could have been tens of dB worse.

In case the relay node 30 would use different frequency channels (oreven strongly different frequencies in case of a proximate frequencychannel operation) for the two links (thereby allowing filtering in casestrongly different frequencies were used), it is natural to not transmitat higher output power than necessary. The same is true in case the twolinks would use different time slots, since then interference would notbe an issue. The present embodiments are therefore concerned with thesituation that interference is an issue and in particular the two linksare using the same or proximate frequency channels (but not necessarilythe same bandwidth), and the power levels are chosen in a beneficial wayto allow for this rather than on an optimal way for the individuallinks.

Although the introduction of the relay node 30 might significantlyreduce the radio requirements, both in terms of the required transmittedpower and the required sensitivity, it might in certain situations stillnot allow for using the same frequency channel or proximate frequencychannels for the two links (at least under normal operating conditions).As indicated in the examples above, the C/I at the RX antenna might insome situations be too low.

In order to further improve the C/I for the signal that eventuallyenters the algorithms used for demodulation in the relay node 30, anactive cancellation of the signal transmitted from the relay node 30 maybe used in addition to the power control features described above. Itshould be noted that the active cancellation aspects described hereincould also be implemented without (e.g., simultaneously) implementingthe power control features presented herein, and vice versa.

In some implementations, active cancellation of self-interference can beviewed as an echo cancellation. The present disclosure is concerned withspecific features of an echo canceller which will substantially improvethe performance in terms of self-interference suppression. In thefollowing embodiments, these features will exemplarily be described whenthe echo canceller is based on the LMS algorithm, but as will be evidentfor anyone with ordinary skill in the art, the features are alsoapplicable to alternative algorithms for echo cancelling, such as forinstance those based on the Recursive-Least-Squares (RLS) algorithm.

FIG. 5 shows a further embodiment of a relay node 30 that is configuredto control suppression of self-interference. FIG. 5 only shows a portionof the receiver chain of the Un transceiver 21 of FIGS. 2A and 2B. Itwill be appreciated that the receiver chain of the Uu transceiver 22 mayhave a similar configuration. It will further be appreciated that theembodiment of the relay node 30 illustrated in FIG. 5 may besupplemented with the components and functionalities discussed abovewith reference to the embodiments illustrated in FIGS. 3 and 4.

As shown in FIG. 5, the receiver chain of the relay node 30 comprises aChannel Selective Filter (CSF), an Analogue-to-Digital Converter (ADC)and digital baseband processing components. In the embodiment of FIG. 5,the digital baseband processing components are merely exemplified by aFast Fourier Transform (FFT) block and the result of the demodulationillustrated by an IQ diagram In practice, the digital baseband willcontain blocks (not shown in FIG. 5) for parameter estimation, forestimating parameters like time, frequency, and channel parameters. Thedigital baseband will also contain blocks (not shown in FIG. 5) forchannel equalization and decoding both concerning the control and datachannels.

The relay node 30 further comprises an interference signal estimator 60having a first input 62 adapted to receive the transmitter signal u fromthe transmitter (denoted by TX in FIGS. 2A and 2B), a second input 64configured to receive adaptation metric, and an output 66 adapted tooutput an estimated interference signal generated by the interferencesignal estimator 60 based on the transmitter signal u and the adaptationmetric. In the present embodiment, the interference signal estimator 60is an adaptive filter that is based on the LMS algorithm. The adaptationmetric may be indicative of the prevailing interference situation.

The relay node 30 also comprises a subtractor 70 coupled to the output66 of the interference signal estimator 60 and configured to subtractthe estimated interference signal from a received signal in the receiverof the relay node 30. In this way, a signal transmitted from the relaynode 30 that leaks back into the receiver of the relay node 30 isactively cancelled so as to suppress self-interference. In the specificembodiment of FIG. 5, the subtractor 70 is configured as an adder and asign inverter coupled before the adder.

In FIG. 6 a flow diagram 600 is depicted that illustrates in a methodembodiment operation of the relay node 30 shown in FIG. 5. In a firststep 602, an interference signal is estimated based on a transmittersignal from the transmitter and based on adaptation metric. Step 602 maybe performed using the interference signal estimator 60 of FIG. 5.

The adaptation metric may be indicative of the success ofself-interference suppression and may thus be used in the context ofself-interference suppression control. If, for example,self-interference suppression is based on an adaptive filter, theadaptation metric may become smaller the better the adaptive filter isconverging. Accordingly, the adaptation metric may be generated based ona processed signal from which the (estimated) interference has beensubtracted. In general, the adaptive filter will converge, so that thefilter coefficients are similar to the channel through which theinterfering signal has leaked. The interference signal may be estimatedby filtering the transmitted signal through the adaptive filter. Thus,the interference situation may be estimated based on the transmittedsignal and the filter coefficients in the adaptive filter.

In a further step 604, the estimated interference signal is subtractedfrom a received signal in the receiver so as to actively cancel a signaltransmitted from the relay node 30 that leaks back until the receiver ofthe relay node 30. In this manner, self-interference can be suppressed.Step 604 may be performed using the subtractor 70 illustrated in FIG. 5.

As shown in FIG. 5, the received signal r consists of the desired signals coming from the base station 10, and the interference which is thesignal u transmitted by the relay node 30 convolved with the impulseresponse h of the channel between the two antennas 25, 26. The receivedsignal r is filtered through the CSF, and then digitized in the ADC forfurther processing in the digital baseband domain.

The reception is assumed to be performed according to prior art, andwhat is of particular interest here is how the effect of the interferingsignal is reduced. Referring to FIG. 5, the LMS algorithm is used toestimate the interference to be subtracted. Since the transmitted signalis known, the interference will be removed when the filter taps in theLMS algorithm correspond to the channel between the antennas 25, 26 plusother effects in the transmitter and receiver chains. In the specificembodiment of FIG. 5, these effects are assumed to all be in the CSF.

An example of how active cancellation of the self-interference canimprove the quality of the received signal is depicted in the diagram ofFIG. 7. FIG. 7 shows simulations of how the effective SNR in the IQdiagram varies as the LMS algorithm adapts.

The simulations of FIG. 7 were done for the DL of LTE, which uses OFDM.The LMS algorithm updates the channel estimate for every OFDM symbol,but not within an OFDM symbol. The (adaptation) metric used to judge theperformance of the LMS algorithm is the SNR in the IQ diagram as will bedescribed below. Another metric could have been used instead, and thechoice of metric is not essential for the invention but merely used tomore easily illustrate the invention. In the simulations, the desiredsignal is only affected by additive white Gaussian noise (AWGN) and thetime and frequency estimation are assumed to be ideal. In practice,there will also be channel estimation etc. for the desired signal, butthis is not relevant to describe the essence of the technique presentedherein.

With an ideal receiver for the desired signal, the SNR obtained in theIQ diagram will be determined only by the AWGN in case the interferencewould be perfectly cancelled. With non-ideal cancellation, the SNR inthe IQ diagram will be a good measure of how much of the interferenceremains after cancellation has taken place. Thus, an upper bound forwhat can be achieved is given by the SNR.

Referring to FIG. 7, the SNR is 20 dB, and it can be seen that the LMSalgorithm can improve the performance from −20 dB (which is C/I withoutcancellation) to 17 to 18 dB with a suitable choice of an adaptationparameter in the LMS algorithm. In this case C/I=−20 dB at the input tothe LMS algorithm.

In the diagram of FIG. 8, the C/I at the input to the LMS algorithm is−40 dB. As can be seen, in order to reach a SNR in the range of 15 dB,the LMS algorithm needs to be updated using very small steps. This needin turn implies that the convergence will be substantially slower.

In FIGS. 7 and 8, the channel model for the leakage between the twoantennas 25, 26 of the relay node 30 was an exponentially decayingchannel with an RMS delay spread of 10 ns. For the case of C/I=−20 dB(see FIG. 7), the LMS algorithm was used with a filter of length 5,i.e., the channel response was estimated by a filter with 5 taps sampledat 30.72 MHz. This was sufficient to suppress the interference withroughly 37 to 38 dB (C/I improved from −20 dB to around 18 dB). For thecase when C/I=−40 dB at the input to the LMS algorithm (see FIG. 8), itwas found that 5 taps did not suffice to come close to the ideal SNR=20dB in the IQ diagram. Instead, the number of taps used for estimatingthe channel was increased to 15, which is what has been used for thesimulations of FIG. 8.

The need for more taps when needing to suppress interference with alarger amount can be explained by the fact that the channel simply needsto be estimated more accurately. The slower convergence can at leastpartly be explained by the fact that a more complex (longer) filter isrequired to reach the desired suppression. In one embodiment, thecomplexity of the algorithm for interference cancellation is thereforeadapted based on the amount of suppression needed.

Since the relay node 30 typically is stationary, the channel variationscan be expected to only be minor. Therefore, an increase or decrease inthe amount of suppression needed can to a large extent be based on acorresponding increase or decrease in the transmit power, or a decreaseor increase in the receive power. Specifically, if it is found that thesuppression is not sufficient, the relay node 30 may request the basestation 10 to increase its transmit power (so that the receive power atthe relay node 30 is increased also). If that is not possible, or theincrease is not sufficient, the relay node 30 may itself decrease thetransmit power used for the Uu link. The latter might typically implythat the supported data rate is reduced correspondingly.

In another embodiment, the fact that a shorter filter used forestimating the channel will converge faster is exploited. Specifically,when channel variations are detected (for instance based on the factthat taps in the filter change more rapidly or indirectly from that factthat the interference level in the demodulation process is increased),the complexity of the filter adapted by the LMS algorithm is reduced inorder to speed up the tracking performance. Then, when the taps in thefilter start to change more slowly, the number of taps in the filtermight again, if found necessary, be increased to allow for increasedsuppression.

To be able to suppress the interference, it is desirable that theinterfering signal does not cause any degradation before it can besubtracted. One requirement is, for instance, that the interferer doesnot saturate the ADC. In the scenarios of FIGS. 7 and 8 above, thesimulations were performed with a floating point implementation.Clearly, the requirements on the ADC might be unreasonable if the C/I atthe antenna is too low.

In FIGS. 9 and 10, the effect of a limited resolution in the ADC isshown. In both figures, there are four lines corresponding to floatingpoint operation (the best performance) and an ADC with 10, 8, and 6 bitseach in I and Q (decreasing downwards). As can be seen, in case C/I=−20dB, 8 bits suffice, whereas using only 6 bits gives a noticeabledegradation. It should be emphasized that this degradation is not due tothe fact that the LMS algorithm does not work, it is due to the dynamicrange of the ADC (to a too large extent) being needed for theinterferer, so that the desired signal will experience increasedquantization noise. When instead C/I=−40 dB, it can be seen that noteven 10 bits are sufficient. Since all these simulations are generatedassuming that the ADC is used optimally (full range is used but noclipping), in a practical situation where Automatic Gain Control (AGC)is used to control the signal level into the ADC, it can be expectedthat at least 1-2 more bits are needed.

When trying to suppress the interference with as much as, for example,40 dB, there are also practical issues to be addressed, as will now beexplained with reference to the relay node embodiments of FIGS. 11 to17. The following embodiments will focus on the DL operation of therelay node 30, which involves the receiver of the Un transceiver 21 aswell as the transmitter of the Uu transceiver 22 in FIGS. 2A and 2B. Itwill be readily apparent that similar operations as discussedhereinafter could be carried out by the relay node 30 in the ULdirection. Moreover, it will be readily apparent that the embodimentsdescribed hereinafter may be readily combined with the power controlfeatures discussed above with reference to the embodiments illustratedin FIGS. 3 and 4. In the following description, similar components willbe denoted with the same reference numerals.

In the discussion of the diagrams of FIGS. 7 to 10, that substantiallyreferred to the relay node embodiment of FIG. 5, it was assumed that theinterference could be modeled as the transmitter (baseband) signal upassing through an interference signal estimator using, for example, theLMS algorithm. While the self-interference cancellation is based on thetransmitter signal as known in the digital domain (u), the actual signalbeing coupled to the RX antenna will differ quite substantially (e.g.,with error contributions of some −35 to −40 dBc) due to accumulation ofvarious non-ideal properties of the transmitter chain. To discuss thesenon-ideal properties further, FIG. 11 illustrates a more detailedembodiment of the relay node 30 that is based on the embodiment of FIG.5.

As illustrated in FIG. 11, the relay node 30 comprises an interferencesignal estimator 60 and a subtractor 70 in the receiver chain asdiscussed with respect to FIG. 5. Additionally, a radio front end 72coupled between a Un antenna 25 and the subtractor 70 in the receiverchain is provided. The radio front end 72 may comprise the CSF and ADCshown in FIG. 5. The receiver chain further comprises a digital basebandprocessing stage 74 that stretches into the transmitter chain. Thedigital baseband processing components of the receiver chain maycomprise the FFT and the demodulator illustrated in FIG. 5.

The transmitter chain of the relay node 30 comprises adigital-to-analogue converter (DAC) 76, one or more analogue filters 78,an IQ modulator 80 as well as a power amplifier 82 that is coupled to aUu antenna 26.

In the embodiment of the relay node 30 illustrated in FIG. 12corresponds to the embodiment of FIG. 11 but while the embodiment ofFIG. 11 is provided for transmitting and receiving using the samefrequency channel, the embodiment of FIG. 12 is provided for proximatefrequency operation. Before entering the interference signal estimator60, the transmitter signal will in the case of proximate frequencychannel operation be passed through an optional frequency translationblock 84 (not illustrated in FIG. 5). The purpose of the frequencytranslation block 84 is to shift the transmitter signal u such that itreflects the difference in Local Oscillator (LO) frequency used for thereceiver and transmitter in case of proximate frequency channeloperation.

In the embodiment illustrated in FIG. 12, the digital basebandrepresentation of the transmitter signal u would have no or very littleco-channel interference contribution to the channel of the receivedsignal r unless there was an overlap between the transmitter andreceiver channel. The (only) benefit from the architecture in FIG. 12would be the reduction of the blocking level. Any block in the receiverafter the subtraction of the transmitter signal estimate by thesubtractor 70 would thus benefit from reduced dynamic rangerequirements.

Regarding the embodiments of FIGS. 11 and 12, the DAC operation can beassumed to provide insignificant levels of errors, especially comparedto the filters 78 that follow and that introduce a droop and possiblyin-band ripple. By itself, this might not contribute to new spectralemissions, but altering the transmit signal will have implications onthe distortion generated later in the transmitter chain. Also, thefilters 78 may introduce significant frequency-dependent IQ imbalanceand DC offset.

The quadrature modulator 80 will also introduce IQ imbalance and DCoffset, but typically not with a frequency dependency up to the extentof the filters 78. Furthermore, the power amplifier 82 will produce asignificant amount of in-band as well as out-of-band non-lineardistortion to be able to operate with reasonable power efficiency.

These errors can to some extent be mitigated by compensation in thedigital baseband domain and/or, in some cases, by calibration of theblocks in the transmitter chain, in particular of the quadraturemodulator 80. Thus, if the capability of the interference signalestimator 60 and subtractor 70 is such that the leakage between theantennas 25, 26 can be suppressed below the levels of errors introducedin the transmitter chain, then these errors must be modelled, orreplicated, to obtain a more accurate estimate of the signal actuallybeing transmitted. Alternatively, or in combination with suchreplication, some blocks of the relay node 30 may be calibrated toremove the “initial” errors as discussed above.

In the embodiment illustrated in FIG. 13, the errors introduced in thetransmitter chain are replicated, or modelled, in the digital basebanddomain to obtain a better estimate of the transmitter signal u. To thisend, three replication blocks 86, 88, 90 are provided that correspond tothe blocks 78, 80, 82, respectively, in the transmitter chain. It willbe understood that any number of replication blocks can be used toreflect the actual transmitter chain architecture (and the errorsintroduced by that particular architecture) to reach the required levelof self-interference suppression. Accordingly, one or more of the threeblocks 86, 88, 90 illustrated in FIG. 13 may be omitted, and additionalblocks could be added.

The purpose of the replication blocks 86, 88, 90 is to model the errorsthat cause self-interference. Thus, the complexity of the models can besimplified by considering that only errors within the channel of thereceived signal need to be accurately modelled. Furthermore, it may besufficient to have static models in some cases, whereas the models mayneed to be adaptive in other cases.

As for the filter replication block 86, digital replication of analoguefilters is straight-forward when the characteristics of the analoguefilters are known with sufficient accuracy. Analogue filters aretypically of low order, which implies that there are only a few numbersof coefficients to alter if the digital replication filter in the block86 needs to be adapted. In case there is a significant imbalance betweenthe I and Q filters of the IQ branches, the filter replication block 86may be split in two filter blocks, that are configured and/or adaptedindependently.

As for the IQ imbalance replication block 88, digital replication of thequadrature modulator 80 includes modelling of IQ phase and gainimbalance as well as modelling of DC offset (or LO leakage). Modellingand adaptation of IQ imbalance and DC offset are as such known in theart.

Regarding the replication block 90 tackling power amplifiernon-linearities, it is known that the power amplifier 82 is more or lessnon-linear and may also exhibit memory effects that further complicatereplication. Nevertheless, those skilled in the art of behaviouralmodelling of power amplifiers and/or of linearizing of power amplifiersknow that there are many of models and means for adaptation with variousdegrees of complexity that can be used to replicate and track thebehaviour of the power amplifier 82.

A low order polynomial that models the (memoryless) complex-valuedbaseband equivalent of the power amplifier 82 is one of the simplestmodels that can be envisioned. Yet it may model distortion within thefrequency band of interest accurate enough to allow for another 10 to 20dB improvement in self-interference suppression. In case adaptation isrequired, a simple model with a few parameters is preferred, as itallows for faster convergence than with models based on many parameters(and intricate relations).

The embodiment of the relay node 30 illustrated in FIG. 14 is based onthe embodiment of FIG. 13 and additionally comprises the frequencytranslation block 84 discussed above with reference to FIG. 12.Accordingly, while the relay node 30 illustrated in FIG. 13 is adaptedto transmit and to receive simultaneously using the same frequencychannel, the relay node 30 of FIG. 14 may use proximate frequencychannels in this regard.

FIG. 15 shows another embodiment of a relay node 30. In the embodimentof FIG. 15, the relay node 30 is provided with a measurement receiver92. The transmitted RF signal is coupled to the measurement receiver 92via an RF coupler 94 (using the LO frequency of the main receiver, andnot that of the transmitter). The measurement receiver 92 is adapted togenerate equivalent digital baseband components of the transmittedsignal. To this end, the measurement receiver 92 comprises an IQdemodulator 96, one or more analogue filters 98 as well as an ADC 100.The output signal of the ADC 100 will be supplied as the transmittersignal to the input 62 of the interference signal estimator 60.

With the architecture illustrated in FIG. 15, the non-ideal behaviour ofthe measurement receiver 92 may be a limiting factor. While there is nonon-linearity problem with the power amplifier 82 (as the measurementreceiver captures what is actually being transmitted), the IQdemodulator may introduce a IQ imbalance and a DC offset.

FIGS. 11 to 15 exemplify relay node architectures for obtaining anestimate of the transmitted signal to be used for self-interferencesuppression. Other architectures can be envisioned in which thetransmitter signal is taken from another node within the transmitterchain with the corresponding need for replication. For example, the RFoutput signal from the IQ modulator 80 could be used instead of theoutput signal from the power amplifier 82 as illustrated in FIG. 15. Insuch an implementation, only the power amplifier non-linearity wouldhave to be replicated.

In yet another embodiment of the relay node 30 illustrated in FIG. 16,the transmitted signal is not down-converted but instead kept in the RFdomain and input to a vector gain processing block 102. The vector gainoperation (a complex gain or corresponding amplitude gain and phaseshift) performed by the vector gain processing block 102 is intended toprovide a first coarse self-interference cancellation already before theRF receiver frontend 72. Here it is assumed that the channel has onedominant tap for the interference suppression scheme to be effective.

The approach illustrated in FIG. 16 has the advantage of decreasing thedynamic range requirements in the analogue receiver domain (radio frontend 72) from a Low Noise Amplifier (LNA) all the way to the ADC. Theremaining interference may be handled in the digital baseband domainusing any of the previous approaches in terms of where to tap thetransmitter chain for the transmitter signal to be input to theinterference signal estimator 60.

An alternative architecture based on such a concept is illustrated inFIG. 17. In the embodiment of the relay node 30 illustrated in FIG. 17,the output signal of the power amplifier 82 is used both for a firstcoarse cancellation at the LNA input as well as a further cancellationin the digital domain.

A still further embodiment of a relay node 30 is illustrated in FIG. 18.Here, the blocking part and the co-channel interference part (see FIG.2A) are handled independently such that each part has its owninterference signal estimator 60, 60′. The blocking part estimate can beobtained and subtracted from the received signal using any of thepreviously proposed schemes. In a similar manner, a separate path isused for the co-channel interference part, and again the means forobtaining this interference part and subtracting it can be based on anyof the schemes discussed above.

Dividing self-interference cancellation into blocker and co-channelinterference cancellation is particularly advantageous when theseparation of the transmitted and the received signals is comparable orlarger than the bandwidth of the signals. This is due to the fact thatthe processing bandwidth does not need to extend over both signals, butrather the bandwidth of each signal. Yet another advantage with thisapproach is that the baseband representation of the transmitter signal uwill resemble the main lobe of the transmitter signal, and thus willsuffice as far as blocker reduction is concerned (i.e., an accurateestimate of the transmitted signal with the deficiencies introduced bythe non-ideal transmitter is not required to reduce the level of themain lobe by several tens of dB).

In the event that one or more of the replication blocks need to beadapted to better model the blocks in the transmitter chain, it needs tobe coordinated or coupled with the execution of the interference signalestimator (i.e., the LMS algorithm) if the same or a related metric(e.g., SNR) is used as guidance for adaptation. The same applies to thecase when one or more blocks in the transmitter chain and/or blocksconveying the transmitter signal back to the interference signalestimator 60 (e.g., the measurement receiver 92) need to be adapted orcalibrated. It is therefore proposed to run the LMS or any otheralgorithm first for a predefined amount of time or until a terminationcondition has been fulfilled (e.g., no further improvement in C/I can bedetected). This step is then followed by a block adaptation/calibration,also running for a predefined amount of time or until a terminationcondition has been fulfilled. This sequence is then repeated byreturning to the execution of the LMS or other algorithm.

It will be increasingly common to exploit multi-antenna techniques suchas MIMO and beam-forming. For such techniques, each receiver mightcomprise one or more of the components taught herein (such as thecanceller) for each transmitter being coupled back to that receiver.

As has become apparent from the above description of exemplaryembodiments, the technique presented herein allows for an efficientself-interference suppression control in a relay node. In this regard,active interference cancelation techniques may be employed. Availablepower margins may be exploited for a power control that aims atcontrolling self-interference suppression.

While the technique presented herein has been described with respect toparticular embodiments, those skilled in the art will recognize that thepresent invention is not limited to the specific embodiments describedand illustrated therein. It is to be understood that this disclosure isonly illustrative. Accordingly, it is intended that the invention belimited only by the scope of the claims appended hereto.

The invention claimed is:
 1. A method of controlling suppression of self-interference in a relay node configured to transmit and receive simultaneously using one of a same frequency channel and proximate frequency channels, the method comprising: actively cancelling a signal transmitted from the relay node that leaks back into a receiver (RX) of the relay node to suppress self-interference, the active cancellation being based on adaptive filtering; determining an amount of suppression needed; adapting a complexity of the adaptive filtering based on the amount of suppression needed; determining when self-interference suppression by active cancellation is insufficient; when insufficiency of self-interference suppression by active cancellation is determined, then signalling a base station to increase power transmitted by the base station to the relay node; determining that the increase of the power transmitted by the base station is one of not possible and not sufficient; and when the increase of the power transmitted by the base station is one of not possible and not sufficient, then decreasing a transmit power of the signal transmitted from the relay node.
 2. The method of claim 1 further comprising requesting an increase of a transmit power of a signal received by the relay node.
 3. The method of claim 2, further comprising: determining an amount of suppression after a previous increase of the transmit power of the relay node does not meet a threshold; and requesting an increase of the transmit power of the signal received by the relay node in response to determining that the amount of suppression does not meet the threshold.
 4. The method of claim 1, further comprising decreasing the transmit power for a link to a user terminal, the transmit power of a signal received by the relay node from a base station not being reduced.
 5. The method of claim 1, further comprising: decreasing the transmit power for a link to a user terminal; and requesting an increase of a transmit power of a signal received by the relay node from a base station.
 6. The method of claim 1, further comprising decreasing the transmit power for a link to a base station, the transmit power of a signal received by the relay node from a user terminal not being reduced.
 7. The method of claim 1, further comprising reducing a data rate supported by the relay node in correspondence with decreasing the transmit power of the relay node.
 8. The method of claim 1, wherein active cancellation is performed by an echo cancellation technique.
 9. The method of claim 1, wherein active cancellation includes estimating the self-interference and subtracting the estimated self-interference.
 10. The method of claim 1, wherein adapting the complexity of the adaptive filtering includes increasing a number of filter taps for a channel estimation to increase an amount of suppression.
 11. The method of claim 1, further comprising: detecting channel variations; and reducing a complexity of the adaptive filtering to increase tracking performance.
 12. The method of claim 1, wherein the determining of when self-interference suppression is insufficient is based on a signal quality threshold.
 13. The method of claim 1, wherein the determining of when self-interference suppression is insufficient is based on an available power margin.
 14. A computer program product comprising program code portions stored in a non-transitory storage medium, which when executed by a computing device, cause the computing device to: actively cancel a signal transmitted from a relay node that leaks back into a receiver of the relay node to suppress self-interference, the active cancellation being based on adaptive filtering; determine an amount of suppression needed; adapt a complexity of the adaptive filtering based on the amount of suppression needed; determine when self-interference suppression by active cancellation is insufficient; when insufficiency of self-interference suppression by active cancellation is determined, then signal a base station to increase power transmitted by the base station to the relay node; determine that the increase of the power transmitted by the base station is one of not possible and not sufficient; and when the increase of the power transmitted by the base station is one of not possible and not sufficient, then decrease a transmit power of the signal transmitted from the relay node.
 15. The computer program product of claim 14, wherein the program code is stored on a computer-readable recording medium.
 16. A relay node configured to transmit and receive simultaneously using one of a same frequency channel and proximate frequency channels, the relay node comprising: a canceller configured to actively cancel a signal transmitted from the relay node that leaks back into a receiver (RX) of the relay node to suppress self-interference, the active cancellation being based on adaptive filtering; determine an amount of suppression needed; adapt a complexity of the adaptive filtering based on the amount of suppression needed; a determination component configured to determine when self-interference suppression by active cancellation is insufficient; a requestor component configured to: signal a base station to increase power transmitted by the base station to the relay node when insufficiency of self-interference suppression by active cancellation is determined; determine that the increase of the power transmitted by the base station is one of not possible and not sufficient; and a power control component configured to decrease a transmit power of the signal transmitted from the relay node when the increase of the power transmitted by the base station is one of not possible and not sufficient.
 17. The relay node of claim 16, wherein the canceller is an echo canceller. 